KR102340279B1 - Method for recovering olefins from solution polymerization process - Google Patents

Abstract

the present invention
(A) passing the solution stream into a separator in which a liquid phase and a gas phase comprising a polymer coexist;
(B) recovering a vapor stream and a concentrated solution stream from the separator;
(C) passing at least a portion of the vapor stream to a first classifier;
(D) recovering a first overhead stream and a first bottoms stream from the first classifier;
(E) passing the first overhead stream to a second classifier;
(F) recovering a second overhead stream and a second bottoms stream from the second classifier;
(G) passing the second overhead stream to a third classifier; and
(H) recovering a third overhead stream and a third bottoms stream from the third classifier;
and at least a portion of the third bottoms stream is recovered as a purge stream.

Description

Method for recovering olefins from solution polymerization process

The present invention relates to a solution polymerization process. More particularly, the present invention relates to the separation and removal of inert components of a reaction mixture downstream of a polymerization process.

It is known to produce an olefin polymer in a solution polymerization process, wherein unreacted monomers, comonomers and solvent are separated from solution and recycled to the polymerization process.

WO-A-2009/013217 discloses a process for separating a feedstream downstream comprising hydrocarbons in an olefin polymerization process. Separation of olefinic monomers, comonomers and hydrocarbon diluents is described.

WO-A-2009/090254 discloses a process for recovering unreacted monomers from a slurry or gas phase polymerization process.

Despite the prior art, there is still a need for an efficient process for separating and recovering inactive components derived from comonomers of polymer solutions in solution polymerization. For example, 1-hexene contains small amounts of hexane and isomers of hexene that are not active in polymerization. Therefore, these components begin to accumulate in the process, reducing their effectiveness. Polymer density control becomes difficult if inactive components are not removed. The present invention provides an efficient method for isolating and removing _{inactive C 6 components.}

the present invention

(A) passing the solution stream into a separator in which a liquid phase and a gas phase comprising a polymer coexist;

(B) recovering a vapor stream and a concentrated solution stream from the separator;

(C) passing at least a portion of the vapor stream to a first classifier;

(D) recovering a first overhead stream and a first bottoms stream from the first classifier;

(E) passing the first overhead stream to a second classifier;

(F) recovering a second overhead stream and a second bottoms stream from the second classifier;

(G) passing the second overhead stream to a third classifier;

(H) recovering a third overhead stream and a third bottoms stream from the third classifier;

and at least a portion of the third bottoms stream is recovered as a purge stream.

1 shows a flow diagram of a fractionation process in which _{inert C 6 components are removed through a purge.}

The present invention relates to a process for polymerizing one or more olefins in solution in one or more polymerization reactors. The solution polymerization process is typically carried out in a solvent in which the monomer, the final comonomer, the final chain transfer agent and the polymer formed during the process are dissolved. Such processes are disclosed inter alia in WO-A-1997/036942, WO-A-2006/083515, WO-A-2008/082511 and WO-A-2009/080710.

polymerization

Polymerization may be carried out in one or more polymerization reactors. If reference is made to one polymerization reactor herein, it is clear that this applies equally to a plurality of reactors, and in particular to any one of said reactors.

In the polymerization reactor, an olefinic monomer having two or more carbon atoms, one or more catalyst systems, optionally one or more comonomers, optionally one or more chain transfer agents, and optionally one or more diluents or solvents are used to carry out the polymerization. Accordingly, the polymerization system is in a dense fluid state and includes the olefin monomer, any comonomer present, any diluent or solvent present, any chain transfer agent present, and the polymer product.

The olefin monomer is an alpha-olefin having at least 2 carbon atoms, preferably an alpha-olefin having from 2 to 10 carbon atoms. More preferably the olefinic monomer is selected from the group consisting of ethylene, propylene and 1-butene. Particularly preferably the olefinic monomer is ethylene.

One or more comonomers are optionally and preferably present in the polymerization reactor. Comonomers include alpha-olefins different from olefinic monomers having 2 to 10 carbon atoms; polyenes such as non-conjugated alpha-omega-diene having 4 to 10 carbon atoms, cyclic olefins having 6 to 20 carbon atoms and cyclic polyenes having 6 to 20 carbon atoms. Preferably, the comonomer is selected from 1-butene, 1-hexene and 1-octene when the olefinic monomer is ethylene; and alpha-olefins different from olefinic monomers having from 2 to 10 carbon atoms such as ethylene, 1-butene and 1-hexene when the olefinic monomer is propylene.

1-Hexene is one of the comonomers typically used in solution polymerization. 1-Hexene typically contains 2-3% of hexane and isomers of hexene, eg iso- or tertiary-hexene or n-hexane, which are not active, for example, for polymerization. Since these components are inert to polymerization, they begin to accumulate during the process.

The polymerization catalyst can be any catalyst known in the art capable of polymerizing monomers and optional comonomers. Accordingly, the polymerization catalyst may be a Ziegler-Natta catalyst as disclosed in EP-A-280352, EP-A-280353 and EP-A-286148, or WO-A-1993025590, US-A-5001205, WO-A - a metallocene catalyst as disclosed in -1987003604 and US-A-5001244, or a combination thereof. In addition, other suitable catalysts may be used, such as late transition metal catalysts.

Chain transfer agents can be used to control the molecular weight of the polymer as is known in the art. A suitable chain transfer agent is, for example, hydrogen.

The solvent is suitably present for the polymerization process. The solvent can be any suitable straight-chain or branched alkyl having 7 to 20 carbon atoms, cyclic alkyl optionally having an alkyl substituent, or aryl optionally having an alkyl substituent, or a mixture of two or more of the compounds listed above. . The solvent must be inert towards the polymerization catalyst and the monomers. In addition, the solvent must be stable under polymerization conditions. In addition, the solvent must be capable of dissolving the monomer, the final comonomer, the final chain transfer agent and the polymer under polymerization conditions.

The temperature of the polymerization reactor is such that the polymer formed in the polymerization reaction is completely dissolved in the reaction mixture comprising the solvent, comonomer(s), chain transfer agent and polymer. The temperature is preferably higher than the melting temperature of the polymer. Accordingly, when the polymer is a homo- or copolymer of ethylene, the temperature is preferably between 120° C. and 220° C., for example between 150° C. and 200° C., depending on the content of comonomer units in the polymer. If the polymer is a homo- or copolymer of propylene, the temperature is preferably between 165° C. and 250° C., for example between 170° C. and 220° C., depending on the content of comonomer units in the polymer.

The pressure in the polymerization reactor depends on the one hand on the temperature and on the other hand on the type and amount of comonomer. The pressure is suitably 50 to 300 bar, preferably 50 to 250 bar, more preferably 70 to 200 bar.

The residence time is short and is usually less than 10 minutes.

The process operates properly continuously. Thereby a stream of monomers, catalyst, and, if present, of comonomer, chain transfer agent and solvent are delivered to the polymerization reactor. A product stream comprising unreacted monomer, dissolved polymer, final unreacted comonomer, chain transfer agent, and final solvent is withdrawn from the reactor.

recovery of ingredients

A product stream is withdrawn from the polymerization reactor continuously or intermittently, preferably continuously. The product stream is then passed to a separation stage in which a liquid phase and a gas phase comprising the polymer coexist.

A product stream may be separated at any process step where volatile compounds may be recovered from solution. Typically such process steps include reduced pressure and preferably heating of the solution. A typical example of such a process step is flashing (instant vaporization, flashing). For example, the product stream is heated and then passed along a pipe to a receiving vessel operated at a pressure substantially lower than the pressure of the polymerization reactor. Thereby, a portion of the fluid contained in the solution is evaporated and recovered as a vapor stream. A portion remaining in solution with the polymer forms a first concentrated product stream.

Preferably the product stream is heated to produce a heated stream. Typically, the temperature of the heated stream is between 200 °C and 300 °C, preferably between 210 °C and 270 °C, more preferably between 210 °C and 250 °C. Preferably the temperature of the heated stream is 10° C. to 120° C., more preferably 20° C. to 100° C. higher than the temperature of the solution in the polymerization reactor.

The pressure of the product stream is reduced to be in the range of 1 to 15 bar, preferably 2 to 12 bar, more preferably 5 to 10 bar. The pressure is preferably reduced to at least about 40 bar to about 295 bar lower than the pressure in the polymerization reactor.

In a preferred embodiment, the separating step is a flashing step. Thereby, a liquid phase and a gas phase exist in the separation step. The flashing step is suitably carried out in a flashing vessel, which is preferably a generally cylindrical vertical vessel. The flashing vessel thereby has a section with an approximately circular cross-section. Preferably the flashing vessel has a cylindrical section having the shape of a circular cylinder. In addition to the cylindrical section, the flashing vessel may have additional sections such as a lower section which may be conical and an upper section which may be hemispherical. In addition, the flashing vessel may also have a generally conical shape.

The temperature of the flashing vessel is typically 130 to 250°C. The temperature should be high enough to maintain the viscosity of the solution at an appropriate level, but below the temperature at which the polymer will decompose. The pressure in the flashing vessel is typically from 15 bar to atmospheric pressure, or even below atmospheric pressure.

The product stream enters the overhead flashing vessel. The solution moves downward in the flashing vessel while the gases evaporating from the solution move upward. According to this preferred embodiment, the polymer solution forms a thin film that drips downward from the flashing vessel. This facilitates the removal of hydrocarbons from the polymer solution. Gas is typically withdrawn from the top of the flashing vessel, while solution is withdrawn from the bottom.

According to a particularly preferred embodiment, the product stream is sprayed into the flashing vessel. Spraying may be performed using one or more suitable nozzles that disperse the solution stream into droplets. Such nozzles are well known in the art and include air spray nozzles, flat fan nozzles, hollow cone nozzles and full cone nozzles. Preferably, the nozzle breaks the stream into droplets having a size of about 1 mm or less.

The nozzle forms a stream of droplets in the flashing vessel. The droplet stream then solidifies in the flashing vessel to form a falling film having a relatively high surface area. This enhances the mass transfer of volatile components from solution.

As noted above, the flashing vessel may have a generally vertical cylindrical shape. The droplet stream is then directed tangentially with the wall of the flashing vessel by appropriate positioning of the nozzle. Thus, the nozzle is positioned relatively close to the wall so that the outlet is directed tangentially to the wall. As the droplet stream exits the nozzle, it travels in the direction of the wall, forming a downwards falling film. The flashing vessel may have a generally vertical conical shape. In this embodiment, it is possible to direct the droplet stream tangentially to the wall of the flashing vessel, as described above. However, it is also possible to direct the droplets axially towards the wall of the flashing vessel. The nozzle or nozzles are then disposed off-center within the flashing vessel. In both configurations, the polymer solution forms a drop film within the flashing vessel.

The polymer content of the first concentrated product stream recovered from the flashing step is typically between 35 and 99 weight percent. In other words, the first concentrated product stream recovered in the first flashing stage contains from 1 to 65 weight percent residual hydrocarbons.

Viewed from a different angle, the hydrocarbon stream recovered from the flashing vessel is 35 to 80 weight percent of the total material stream recovered from the flashing vessel. The hydrocarbon stream typically comprises unreacted monomers and solvents and unreacted comonomers.

High separation efficiency can be achieved by using flashing as described above. For example, the separation efficiency for hydrocarbons containing 6 carbon atoms is 75% or more, preferably 80% or more. Further, the separation efficiency of hydrocarbons containing 8 carbon atoms is 60% or more, preferably 65% or more. Separation efficiency is defined as the mass flow of the components recovered from the vapor stream divided by the (theoretical) mass flow rate of the components in the vapor stream at equilibrium.

The first concentrated product stream contains unreacted comonomer and polymer dissolved in solvent. It may also contain residual monomers still remaining in solution. Typically, the polymer concentration in the first concentrated product stream is from 40% to 90% by weight, preferably from 50 to 80% by weight, most preferably from 60 to 75% by weight, based on the total weight of the first concentrated product stream. . The first concentrated product stream is then typically liquid. However, the first concentrated product stream may contain minor amounts of vapors such as vapor bubbles. The amount of vapor in the first concentrated product stream is typically no more than 40% by volume, preferably no more than 30% by volume, particularly preferably no more than 20% by volume, for example no more than 10% by volume or no more than 5% by volume.

The vapor stream contains unreacted monomers and other volatile compounds such as hydrogen. The vapor stream may also contain some solvents and comonomers. The vapor stream may optionally include small amounts of liquid droplets. The amount of these droplets is typically 40% by volume or less, preferably 30% by volume or less, particularly preferably 20% by volume or less.

The first concentrated product stream is then passed to a subsequent process step. Preferably the subsequent process steps comprise at least one further separation step, more preferably at least two further separation steps. This further separation step can be carried out, for example, in a manner analogous to the separation step described above. A further vapor stream is withdrawn from the further separation stage.

In a preferred embodiment of the invention, the product stream is passed to three subsequent flashing stages.

A portion of the vapor or further vapor stream from any separation and further separation stages may be passed to a fractionation stage, preferably a first fractionator.

The solution stream is hereinafter the product stream recovered from the polymerization reactor, the first concentrated product stream recovered from the first separator, the second concentrated product stream recovered from the second separator or any other concentrated product stream recovered from any subsequent separator. indicates

A separator is hereinafter referred to as a first separator, a second separator, a third separator or any subsequent separator.

The concentrated solution stream refers to the solution stream recovered from the separator hereinafter.

Vapor stream refers hereinafter to any vapor or additional vapor stream or a combination thereof.

In the present invention, a solution stream comprising 1-hexene and a solvent comprising at least 7 carbon atoms, preferably n-octane, is passed to the separator. In the separator, a liquid phase and a gas phase comprising the polymer coexist. A concentrated solution stream and a vapor stream are withdrawn from the separator. At least a portion of the vapor stream is passed to a first classifier. A portion of the vapor stream from any of the above separation stages may also be passed to the first fractionator.

A first overhead stream and a first bottoms stream are withdrawn from the first classifier. The first classifier may be any device from which the first overhead stream may be separated from the first bottoms stream. A distillation column or a stripping column is preferred. The first bottoms stream comprises hydrocarbons having at least 9 carbon atoms and a solvent. The first overhead stream comprises solvent, monomer and nitrogen.

The first overhead stream from the first classifier is passed to the second classifier. The second fractionator may be any device suitable for separating a second overhead stream from the second bottoms stream, for example a distillation column, a stripping column, a multi-phase fractionator, an extractor or a liquid-liquid separator. A second overhead stream and a second bottoms stream comprising monomers and 1-hexene are withdrawn from the second fractionator.

A second overhead stream from the second classifier is passed to a third classifier. The third classifier may be any device suitable for separating a third overhead stream from a third bottoms stream. The third classifier is preferably a distillation column or a stripping column. A third overhead stream comprising monomers and a third bottoms stream comprising 1-hexene are withdrawn from the third fractionator. A portion of the third bottoms stream may be passed to the polymerization reactor via a feed vessel. A portion of the third overhead stream may be passed to the polymerization reactor via a feed vessel. The third bottoms and overhead streams may be subjected to additional steps such as purification before entering the feed vessel.

A portion of the third bottoms stream is recovered as a purge stream. _{Inert C 6} components such as isomers of n-hexane and 1-hexene are removed from the purge stream. The purge stream can be recovered and used further as gasoline. It can also be used as a fuel additive. The value of the purge stream may be increased by isomerization and/or hydrogenation. In a preferred aspect of the invention, the purge stream is recovered and used further. In a more preferred aspect of the invention, the purge stream is recovered and isomerized and/or hydrotreated for further use.

The first bottoms stream from the first classifier is passed to a fourth classifier. The fourth classifier may be any device suitable for separating a fourth overhead stream from a fourth bottoms stream. The fourth classifier is preferably a distillation column or a stripping column. A fourth overhead stream comprising solvent and a fourth bottoms stream comprising hydrocarbons having at least 9 carbon atoms are withdrawn from the fourth fractionator. A portion of the fourth overhead stream may be passed to the polymerization reactor via a feed vessel. The fourth overhead stream may be subjected to additional steps such as purification before entering the feed vessel.

Advantages of the present invention

The inert C ₆ component is separated and removed from the process and therefore does not accumulate in the process. When inert components are removed, the energy consumption and operating costs of the process are reduced due to the reduced amount of recycle. These components may be further used as such or may be treated, for example by isomerization or hydrogenation.

Description of drawings

1 is an exemplary diagram of a process in which ethylene is polymerized with 1-hexene in a solvent containing 7 or more carbon atoms. The inert C ₆ component is removed via a purge.

1 , the solution stream is delivered to flashing (not shown). In flashing, a liquid phase and a gas phase containing a polymer coexist. A concentrated solution stream and vapor stream 32 are recovered from the flashing. A portion of vapor stream 32 is passed to first distillation column 25 . There may be more than one flashing before flashing. For two or more subsequent flashing, a portion of each vapor stream is passed to the first distillation column. A first overhead stream (36) and a first bottoms stream (33) are recovered from the first distillation column (25). A first overhead stream (36) is passed to a second distillation column (27). A second overhead stream (37) and a second bottoms stream (38) are withdrawn from the second distillation column (27). A second overhead stream (37) is passed to a third distillation column (28). A third overhead stream 44 and a third bottoms stream 39 are withdrawn from the third distillation column 28 . A portion of the third bottoms stream 39 is withdrawn from the purge stream 40 . _{Inert C 6} components such as isomers of hexene are removed via purge stream 40 . The remainder of the third bottoms stream 39 comprising 1-hexene may be passed to the feed vessel 30 via line 41 . A third overhead stream 44 comprising ethylene may be passed to compressor 31 , line 45 and feed vessel 30 . A portion of the third overhead stream 44 may be recovered as another purge stream 48 . Components such as ethylene and nitrogen may be removed via the purge stream 48 . The first bottoms stream 33 is passed to a fourth distillation column 26 . A fourth overhead stream (34) and a fourth bottoms stream (35) are withdrawn from the fourth distillation column (26). The fourth bottoms stream 35 comprises components such as hydrocarbons having 10 or more carbon atoms. A fourth overhead stream (34) comprising solvent may be delivered to a feed vessel (30). A feed stream is withdrawn from the feed vessel (30). The feed stream is passed through a cooling step (not shown) to a polymerization reactor (not shown).

Example

Computer simulations were performed using Aspen 8.8 computer software. In the simulations, ethylene and 1-hexene were polymerized in n-octane in a polymerization reactor. A product stream was withdrawn from the polymerization reactor. The product stream was subjected to three subsequent flashing stages. A portion of the vapor stream from each flashing stage was passed to the first fractionator. The overhead stream from the first classifier was passed to the second classifier. The overhead stream from the second classifier was passed to the third classifier. In an embodiment of the present invention, a portion of the bottoms stream from the third fractionator is withdrawn from line 40 as a purge stream. In the comparative example, a portion of the overhead stream from the first classifier was withdrawn from line 36 as a purge stream. The purge streams at different locations are compared in Table 1.

Fudge stream

CE
IE
Total mass flow (kg/h)

176.8
175.8
Ethylene (kg/h)

1.0
0
Ethane (kg/h)

0.2
0
1-Hexene (kg/h)

80.6
79.0
iso-hexene ^* (kg/h)

32.6
31.9
n-octane (kg/h)

62.3
64.8

* Isomers in which the double bond is not in the 1-position, eg trans-2-hexene, trans-4-methyl-2-pentene and 2-ethyl-2-butene. Isomers of hexane may also exist.

In the above comparison, _{it can be seen that the C 6} and C ₈ flows are almost similar in IE and CE. However, there is no ethylene and no ethane in IE. This is advantageous since no separation of _{C 2} is required when the purge stream is withdrawn from line 40 .

Claims (12)

(A) passing the solution stream into a separator in which a liquid phase and a gas phase comprising a polymer coexist;
(B) recovering a vapor stream and a concentrated solution stream from the separator;
(C) passing at least a portion of the vapor stream to a first classifier;
(D) recovering a first overhead stream and a first bottoms stream from the first classifier;
(E) passing the first overhead stream to a second classifier;
(F) recovering a second overhead stream and a second bottoms stream from the second classifier;
(G) passing the second overhead stream to a third classifier;
(H) recovering a third overhead stream and a third bottoms stream from the third classifier;
at least a portion of the third bottoms stream is recovered as a purge stream.
According to claim 1,
wherein the solution stream comprises 1-hexene, n-octane and a mixture of isomers of 1-hexene and n-hexane.
3. The method of claim 2,
purging the isomers of 1-hexene and n-hexane with a purge stream.
4. The method according to any one of claims 1 to 3,
(I) passing the first bottoms stream to a fourth classifier;
(J) recovering a fourth overhead stream and a fourth bottoms stream from the fourth classifier;
(K) optionally passing at least a portion of the fourth overhead stream to a polymerization reactor.
4. The method according to any one of claims 1 to 3,
(L) optionally passing at least a portion of the third overhead stream to a polymerization reactor.
4. The method according to any one of claims 1 to 3,
(M) optionally passing at least a portion of the third bottoms stream to a polymerization reactor.
5. The method of claim 4,
(N) optionally passing at least a portion of any of said third bottoms stream, third overhead stream or fourth overhead stream to a feed vessel;
(O) recovering a feed stream from the feed vessel;
(P) passing the feed stream through a cooling step to a polymerization reactor.
4. The method according to any one of claims 1 to 3,
wherein the solution stream comprises ethylene.
5. The method of claim 4,
wherein any of the first, second, third or fourth classifier is a distillation column, a stripping column, a multiphase separator, an extractor or a liquid-liquid separator.
4. The method according to any one of claims 1 to 3,
and at least a portion of the purge stream is used as a gasoline additive or a fuel additive.
4. The method according to any one of claims 1 to 3,
at least a portion of the purge stream is passed to a hydrogenation step.
4. The method according to any one of claims 1 to 3,
at least a portion of the purge stream is passed to an isomerization step.

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